In a power supply system, as a protective measure, a protective grounding of conductive accessible parts of electrical equipment is an important part of the protective measure “protection by automatic shut-off of the power supply” as required according to standards. This is true irrespective of whether the network type is an ungrounded power supply system (French: Isolé Terre—IT network) or a grounded power supply system (French: Terre Neutre—TN network or French: Terre Terre—TT network).
Hence, the protective grounding of a subsystem of a branched power supply system via a protective conductor connection with the subsystem deserves special attention since in most cases a disconnection of the protective conductor will disable this protection. In this context, a subsystem refers to one unit of an entire power supply system, wherein the unit can be shut off. Said subsystem usually comprises one or more pieces of electrical equipment.
When a disconnection of a protective conductor connection (first fault) is joined by another fault (second fault), such as failure of the basic insulation due to short-circuiting of clearances and creep distances or due to defective insulation, there is an increased risk of electric shock.
Since the risk of this two-fault situation occurring in power supply system is not negligibly small, the use of residual current devices (RCDs) as additional protection has become the norm in grounded power supply systems.
In many industrial power supply systems, however, the use of RCDs as additional protection against electric shock is not possible because due to very large network leakage capacitances present in the power supply systems, even in the absence of an additional residual current, there is a leakage current that can be significantly higher than 30 mA and which would thus instantly trigger any RCDs present in the power supply system.
If it is impossible to use the RCD in a grounded power supply system because of excess leakage currents or if the RCD used is not suitable for protection against electric shock (designed for fire and installation protection only), there is the risk in case of a disconnected protective conductor connection and a second fault that a person operating the equipment as intended will suffer a dangerous electrical accident because the fault circuit will close via the person's body.
In contrast, when the protective conductor connection is intact, the residual current in case of failure of the basic insulation will run almost exclusively through the protective conductor back to the feeding point of the grounded power supply system. However, this will lead to very high ground-fault currents and usually also to contact voltages of dangerously high amplitude—provided the grounded power supply system is designed correctly. For this reason, a grounded power supply system has to be shut off quickly enough in case of a first fault.
The installation of converter systems in grounded power supply systems deserves special attention with regard to protective measures. The protective conductor connection with a converter system is particularly critical because insulation faults at the output of the converter system toward accessible and conductive parts of a converter-controlled drive can lead to residual currents that can exhibit not only network-frequency portions but also a fairly broad spectrum of converter-specific spectral portions from direct-current components to portions in the MHz range.
It also needs to be noted that large leakage capacitances between the output phases of the converter and the drive housing (output filter) can act as a low-impedance connection for the higher-frequency portions.
In this case, conventional type A RCDs do not offer sufficient additional protection. When a protective conductor connection is faulty, touching of the converter-controlled drive can lead to electric shock without a type A RCD recognizing it. Even the use of mixed frequency-sensitive type F RCDs will not reliably prevent the hazard from electric shock in most cases.
During normal operation, the leakage currents in the switch-frequency range of the converter (kHz range) will already be significantly higher than 30 mA in most cases; during normal operation of high-power converter drives, even the leakage-current limit of 300 mA is often exceeded. The use of an RCD—even for reasons of fire protection—is impossible in systems of this kind.
Thus, a reliable protective grounding of the accessible conductive drive components is one of the most important protective measures especially in high-power converter drives.
In an ungrounded power supply system, too, in which by definition all active parts of the power supply system are separated from the ground potential—against ground—and the connected equipment is connected to a grounding installation via a protective conductor, a two-fault situation can become dangerous when a piece of equipment is touched if the power supply system is an extensive ungrounded power supply system having a consequently large total network leakage capacitance. In this two-fault situation, the fault circuit will close via the touching person and the network leakage capacitances.
If the protective conductor connection is intact and the basic insulation is faulty, the residual current will run almost entirely through the protective conductor and through the network leakage capacitances. Even in case of a single fault in an ungrounded power supply system, this will only lead to harmless contact voltages at the equipment. For this reason, an ungrounded power supply system can continue to operate in case of a first fault.
Solutions for addressing the hazard arising from a disconnected protective conductor are known from the state of the art, but they have considerable disadvantages in some parts.
For instance, there are proposals for selective residual current detection that can distinguish between leakage currents and residual currents. However, RCDs on the basis of these ideas are unavailable because reliable functioning in 3-phase alternating-current systems could not be proved so far.
Furthermore, devices are available on the market that are supposed to allow the use of residual current devices for protection against electric shock in industrial systems as well by compensating capacitive leakage currents. However, it is not known how reliably protective means of this kind will work in widely branched industrial networks with changing complex operating states.
Finally, there are loop monitoring devices on the market that monitor a protective conductor connection directly at the equipment. In the presence of a plurality of equipment at different network branches, a corresponding number of loop monitoring devices are needed.
Therefore, the object of the present invention is to provide a method and an electrical protection device which detect in advance, in both grounded and ungrounded branched power supply systems, i.e. in power supply systems provided with units capable of being shut off (subsystems), a disconnection of a protective conductor connection with a subsystem without causing an interruption of operation. In a grounded power supply system, particular significance should be placed on the special case of a converter system connected to the subsystem.